1. Field of the Invention
The present invention relates to an insulated wire coated with a vinyl chloride resin composition free from lead compounds. More particularly, the present invention relates to an insulated wire for automotive vehicles.
2. Description of the Related Art
Conventionally, vinyl chloride resin compositions generally obtained by adding a stabilizer, a lubricant, etc. to a polyvinyl chloride (PVC) have been used as insulation coatings and sheaths of automotive wires due to their suitable flexibility and abrasion resistance. Stabilizers used in the vinyl chloride resins include, for example, tribasic lead sulfate, dibasic lead phosphite, and lead silicate, whereas lubricants used therein include lead stearate. Such lead compounds are frequently used.
When an automotive vehicle is scrapped, wiring harnesses comprised of automotive wires and the like are shredded into dust and buried in the ground. However, since lead compounds contained in the stabilizer and the lubricant are eluted from the buried dust by rainwater, they may cause an environmental pollution. To avoid these problems, there has been an increasing tendency in recent years to use lead-free stabilizers. For example, calcium-zinc stabilizers are used as the lead-free stabilizer. The heat resistance and the weather resistance of the vinyl chloride resin coating have been improved by using hydrotalcite together with the calcium-zinc stabilizer.
However, the vinyl chloride resin mixed with hydrotalcite, adheres more strongly to a copper conductor than prior art wire coatings. This stronger adherence can cause a problem. More specifically, insulation stripping operations are essential to a wiring harness manufacturing operation. Such an intermediate stripping operation involves making a cut in an insulation coating at an intermediate position of the wire and displacing the cut insulation coating to provide a space required for a crimping operation. However, the above-described insulation coating is strongly adhered to the copper conductor and may be torn or cracked during the stripping operation or may corrugate without smoothly moving along the copper conductor during the stripping operation. If such an event occurs during the intermediate stripping operation, a terminal cannot be crimped at the intermediate position of the wire. This, of course, is a critical problem to the wiring harness manufacturing operation.
In view of the above situation, an object of the present invention is to provide an insulated wire having an insulation coating which has an improved heat stability and an excellent strippability.